Plant communities assemble themselves flexibly,and their particular structure depends on the specific history of the area.Ecologists use the term“succession”to refer to the changes that happen in plant communities and ecosystems over time.The first community in a succession is called a pioneer community,while the long-lived community at the end of succession is called a a climax community.Pioneer and successional plant communities are said to change over periods from 1 to 500 years.These changes in plant numbers and the mix of species are cumulative.Climax communities themselves change but over periods of time greater than about 500 years.

An ecologist who studies a pond today may well find it relatively unchanged in a year’s time.Individual fish may be replaced,but the number of fish may tend to be the same from one year to the next.We can say that the properties of an ecosystem are more stable than the individual organisms that compose the ecosystem.

At one time.Ecologists believed that species diversity made ecosystem stable.They believed that the greater the diversity the more stable the ecosystem.Support for this idea came from observation that long-lasting climax communities usually have more complex food webs and more species diversity than pioneer communities.Ecologist concluded that the apparent stability of climax ecosystems depended on their complexity.To take an extreme example,farmlands dominated by a singe crop are so unstable that one year of bad weather or the invasion of a single pest can destroy their entire crop.In contrast,a complex climax community,such as a temperate forest,will tolerate considerable damage from bad

weather or pests.

The question of ecosystem stability is complicated,however.The first problem is that ecologist don’t all agree what“stability“means.Stability can be all defined as simply lack

of change.In that case,the climax community would be considered the most stable,since,

by definition,it changes the least over time.Alternatively,stability can be defined as the speed with which an ecosystem returns to a particular form following a major disturbance,such as a fire.This kind of stability is also called resilience.In that case,climax community would be the most fragile and the least stable,since they can acquire hundreds of years to return to the complex state.

Even the kind of stability defined as simple lack of change is not always associated with maximum diversity.At least in temperate zones,maximum diversity is often found in mid-successional stages,not in the climax community.Once a redwood forest matures,for example,the kinds of species and the number of individuals growing on the forest floor are reduced.In general,diversity,by itself,does not ensure stability.Mathematical models of ecosystems likewise suggest that diversity does not guarantee ecosystem stability-just the opposite,in fact.A more complicated system is,in general,more likely than a simple system to break down.(A fifteen-speed racing bicycle is more likely to break down than a child’s tricycle)

Ecologists are especially interested in knowing what factors contribute to the resilience of communities because climax communities all over the world are being severely damaged or destroyed by human activities.The destruction caused by volcanic explosion of Mount.St.

Helens,in the northwestern United States,for example,pales in comparison to the

construction cause by humans.We need to know what aspects of community are most important to the community’s resistance to destruction,as well as its recovery.

Many ecologists now think that the relative long-term stability of climax communities comes not from diversity but from the“patchiness”of the environment.An environment that varies from place to place supports more kinds of organisms than an environment that is uniform.A local population that goes extinct is quickly replaced by immigrants from an adjacent community.Even if the new population is of a different species,it can approximately fill the niche vacated by the extinct population and keep the food web intact.

2 According to paragraph 1,which of the following is not true of climax communities?A They occur at the end of a succession.

B They last longer than any other type of community.

C The numbers of plants in them and the mix of species do not change.

D They remain stable for at least 500 years at a time.

3 According to paragraph 2,which of the following principles of ecosystems can be learned by studying a pond?

A Ecosystem properties change more slowly than individuals in the system.

B The stability of an ecosystem tends to change as individuals are replace.

C Individual organisms are stable from one year to the next.

D A change in the numbers of an organism does not affect tan ecosystem’s properties.

4 According to paragraph 3,ecologist once believed that which of the following illustrated the most stable ecosystems?

A Pioneer communities

B Climax communities

C Single-crop farmlands

D Successional plant communities

5 According to paragraph 4,why is the question of ecosystem stability complicated?A The reasons for ecosystem change are not always clear.

B Ecologists often confuse the word“stability”with the word“resilience”.C The exact meaning of the word“stability”is debated by ecologists.D There are many different answers to the ecological questions.

6 According to paragraph 4,which of the following is true about climax community?A They are more resilient than pioneer communities.

B They can be considered both the most and the least stable communities.

C They are stable because they recover quickly after major disturbances.

D They are the most resilient communities because they change the least over time.

7 Which of the following can be inferred from paragraph 5 about redwood forest?A They become less stable as they mature.

B They support many species when they reach climax.

C They are found in temperate zones.

D They have reduced diversity during mid-successional stage.

9 In paragraph 5,why does the author provide the information that“a A fifteen-speed racing bicycle is more likely to break down than a child’s tricycle”?

A To illustrate a general principle about the stability of systems by using an every-day example.

B To demonstrate that an understanding of stability in ecosystem can be applied to help understand stability in other situations.

C To make a comparison that supports the claim that,in general,stability increases with diversity.